1. Field of the Invention
This invention relates to dynamic loudspeakers used for the reproduction of sound.
2. Discussion of Prior Art
With the demand for constantly increasing quality in reproduced sound, particularly for musical reproduction, the requirements for low distortion in replicating elements such as loudspeakers has increased as well. This has resulted in the scientific community discovering that certain distortions, previously considered inaudible, can be discerned by the human ear to the detriment of the sound quality. Each improvement in the quality of one component, such as electronic amplifiers, places increasing demands on the requirements for other components in the reproduction chain.
In accordance with current practice, the most common loudspeaker transducer employed in home reproduction systems and many theater installations is the electro-dynamic type, sometimes called simply the dynamic type. This is the most inexpensive kind of speaker. It is rugged and reliable, and its efficiency is acceptably high for home and most theater uses. It can reproduce sound over a large frequency band.
The main limitation of the electro-dynamic speaker is its perceived quality. This is considered inferior to the quality exhibited by other kinds of transducers such as electrostatic speakers, magnetic planar array devices and ribbon speakers. These latter types are considered to produce sound more accurate than that reproduced with dynamic speakers.
The electro-dynamic speaker is used more than any other, because it is far cheaper than any of the speakers that have the reputation of higher quality. The electro-dynamic speaker can also radiate more sound power in a typical installation volume than can any of the kinds of speakers that have a reputation for higher quality. Thus its advantages normally outweigh the dynamic speaker's reputation for inferior quality.
A typical system designed to reproduce substantially all the sound frequencies audible to the human ear will use two or more dynamic speakers. Each speaker will be designed to perform optimally within a specified frequency bandwidth. Each speaker will be supplied frequencies within its frequency band with use of a dividing network or crossover network.
A common system uses a dynamic speaker with a conical radiator 5.5" in diameter to cover the frequencies from 60 Hz to 3,000 Hz. Another speaker with a dome-shaped radiating surface 1.0" in diameter radiates frequencies from 3,000 Hz to 20,000 Hz. The signal from the electronic amplifier that is driving the speaker system is divided into two frequency bandwidths by use of a cross-over network consisting of electrical elements such as inductors, capacitors and resistors. Three or more speakers can be used to cover the entire audio band with the use of more complicated cross-over networks. Each of the three or more speakers radiates sound within a narrower band of frequencies than when only two speakers are used. When electro-dynamic speakers emit sound straight into the room they are called direct radiator speakers.
The electro-dynamic speaker can be made more efficient than it usually is. If this is done the overall clarity and forceful character of the sound reproduced by the speaker will be enhanced. This is mainly because of the influence of voice coil heating. As power is applied to the speaker any inefficiencies are dissipated as heat in the coil. A speaker with more efficiency heats the voice coil less. The added efficiency, however, can only be attained at the expense of a reduction in frequency response bandwidth in a dynamic speaker if the system size is held constant.
The design of any particular electro-dynamic speaker for use in a high quality system is a carefully chosen balance among all the design factors. It is necessary to trade off the parameters of efficiency, clarity, frequency bandwidth, cost and size in any particular design. Enhancement of one consideration can generally be made only at the expense of the others.
The trade off is not a simple one. If the advantages of increased clarity and efficiency are utilized in a particular installation it is necessary to use more speakers to cover the entire audible band than if the speakers had been designed with a lower efficiency but with a correspondingly larger useful bandwidth. The degradation in quality engendered by the use of more crossover network elements and different placements of the sound source as different frequencies are reproduced can overshadow the increase in quality obtained with the added clarity brought about with higher efficiency. More efficient speakers are also larger.